1. Field of the Invention
The present invention relates to an inkjet head for ejecting ink onto a recording medium and an inkjet printer for performing printing operations with the inkjet head.
2. Description of the Related Art
An inkjet head distributes ink supplied from an ink tank to a plurality of pressure chambers to eject ink through nozzles that are in fluid communication with the pressure chambers by selectively applying pressure in pulses to the pressure chambers one of methods for selectively applying pressure to the pressure chambers is to use an actuator unit formed of a plurality of ceramic piezoelectric sheets laminated together.
One inkjet head having this type of actuator unit is disclosed in Japanese unexamined patent application publication HEI-4-341852 (FIG. 1). The inkjet head has a plurality of individual electrodes disposed opposite a plurality of pressure chambers for changing the volume of the pressure chambers in response to drive signals; a common electrode disposed over the plurality of pressure chambers and maintained at ground potential; and a piezoelectric sheet interposed between the individual electrodes and the common electrode. When the individual electrodes are set at a potential different from that of the common electrode to cause an electric field across the polarizing direction of the piezoelectric sheet, the piezoelectric sheet interposed between the individual electrodes and the common electrode and polarized in the laminating direction of the sheets deform in the laminating direction according to a longitudinal piezoelectric effect. If the directions of the electric field and polarization are the same, then the piezoelectric sheet expands in the laminating direction. This deformation of the piezoelectric sheet changes the volume in the pressure chamber, causing ink to eject through a nozzle in communication with the pressure chamber toward a recording medium.
As the density of pressure chambers continues to increase in this type of inkjet head in recent years in order to meet high resolution and high-speed printing needs, a problem called structural cross-talk has arisen. The structural cross-talk means that deformation in the piezoelectric sheet facing a certain pressure chamber accidentally results in deforming another portion of the sheet facing adjacent pressure chambers. As a result, ink may be ejected from nozzles where such ejection is not intended. The amount of ink intended to be ejected from the target nozzle may be changed.
Due to the structural cross-talk, when ink is ejected from that pressure chamber, the amount of deformation in the piezoelectric sheet facing a given pressure chamber may be changed depending on whether ink is simultaneously ejected from neighboring pressure chambers. Accordingly, the amount of ink ejected from this pressure chamber is not stable.